As part of our off-grid solar series, we’re looking for bare-bones options for the charger and inverter components to meet the minimum requirements of driving a 70 watt freezer, 24/7 if possible. We recently tested a Bestek 300W Pure Sine 12V Inverter, shown to the right. Although this unit failed to drive the freezer, it is still worth reviewing in detail. Also, review our previous inverter article in the off-grid solar series for important background information regarding inverters in general.
The unit is small, light and fairly rugged, so we wouldn’t think twice about tossing it in the trunk or the tire well and forgetting about it until it is needed. While we wouldn’t chock the wheels with it, it isn’t a glass slipper, either, but rugged enough to be comforting. The small size and light weight caused us to suspect that it wouldn’t be able to handle the promised surges, though. This turned out to be the case.
When our sponsor SoftBaugh purchased this Bestek unit for us a few weeks ago, it was only about $35 on Amazon. The price is now around $41 as we write this. The combination of price and promised wattage (300 watts continuous, 700 watts peak) should have been a no-brainer for our freezer application, but we quickly encountered several gotchas.
The first gotcha involves the internal fuse/breaker. At 40 amps, the maximum this unit can supply is 480 watts, not 700 watts as per the surge rating unless the surge is very short. In addition, although the wires are fairly beefy, the cigarette lighter plug itself is limiting in terms of current. We would rather that a 300/700 watt unit have terminals on it, and then allow a plug to be wired to that as an option. Given that most automotive cigarette lighter circuits are fused at somewhere between 10 and 15 amps, this limits the maximum power in that mode to 180 watts or so. The wires are fairly beefy, and appear to be 14 AWG (50 strands of 30 AWG wire), although this is not explicitly stated.
To get the maximum out of the unit would require a cigarette lighter adapter. For this purpose, SoftBaugh provided us two NOCO battery adapters. The first adapter, GC017, has clamps, and cost about $8. The other, the GC018, has ring lugs and cost about $7, reflecting the greater cost of the clamps. Regular readers will not be surprised that we prefer the ring lug option for a more reliable connection. The main drawback to the NOCO adapters is that they both use 16 AWG wire, which is a little less beefy than we would like. Both adapters have an in-line 15 amp fuse holder (fuse included), which is a nice feature, but again, limits the inverter power to 180 watts at best.
To test the inverter, we attached it to our tractor while idling. The inverter, with no load, draws about 7 watts, which is a little on the high side but not critical. At no-load, the inverter produces a nice sine wave at 109 volts RMS, which is a little low for our taste. We attempted to run the freezer several times, but tripped the inverter each time. Our tractor is equipped with a relatively fresh battery and has a 90 amp alternator, so there was plenty of juice available at the source.
After this failure, we attempted several lighter loads. One of our main needs for a 12 volt inverter is for work lighting when in the field. For example, a while back we were cutting firewood offsite when the sun set. Not wanting to waste the cool weather, we kept cutting, and really needed some work lights. We had brought along some LED shop lights, one 40 watt and another 20 watt. The inexpensive “funnel” inverter we had at the time, a modified sine unit labeled for 140 watts, was incapable of operating even one of these lights, although the flickering was a nice light show. Apparently, derating inverters to a fraction of their advertised wattage is a good, all-around tip.
The Bestek 300 watt inverter easily handled both of these lights, and the sine wave remained clean throughout. Although a pure sine wave is a waste with lights, we are confident that we could run small motors with it also. We demonstrated this by running a 150 watt box fan with no problem. And, pure sine will be important even with lights if they need to be run at the end of a long string of extension cables, which can cause terrible spikes if used with a modified sine inverter.
Overall, as a pure sine wave inverter in the $30 range, while it was unable to run our 70 watt freezer, we think the Bestek is a fairly good purchase for smaller needs. We are on the fence with it above $40, unless we were able to verify that it could run a newer, more efficient 30 watt to 40 watt freezer, or the new Asian market brushless compressors which are about to be released. The latter can be operated on less power, and lower quality power, than currently available compressors. We would also pick up one or both of the NOCO battery adapters. In any event, having a decent 12 volt pure sine inverter on hand could not hurt, but not with the expectation that it will be able to handle serious loads.
The most important result from this test is a confirmation of two important details:
First, while 12 volt battery arrays are supported by cheaper non-sine inverters and other accessories, for serious inductive demands, we remain committed to 24 volt battery arrays, at a minimum. To get any useful benefit from an off-grid system, you’ll need more than one battery anyway, and once you go there, you might as well configure them as 24 volts and reap the benefits of that higher voltage, where smaller currents and smaller wires aren’t as much of an issue. Even small solar chargers, such as the 10 amp and 20 amp Outback MPPT SmartHarvests, can store twice as much power at 24 volts than they can at 12 (our sponsor SoftBaugh has these MPPT chargers available for much less than Amazon here and here).
Finally, our previous recommendation of the 24 volt MicroSolar 1000 watt pure sine wave inverter, although it should be derated to about 400 to 500 watts from its nameplate value, still stands. Its excellent waveform quality, combined with its reasonable price of about $160 for the pure sine 400 watt inverter, is a good match to our total of about 350 to 400 watts of freezer + refrigerator(s) demand. We’re a little over two weeks into a longevity test with this unit, with no problems thus far which can be attributed to it. Although we haven’t tested the larger 3000 watt version (which should probably be derated to 1000 watts), it is probably priced well at $300.